1200w PSU? Why so much power, are you planning on running 3 GTX 580s?
Anyway, for BF3 I have read a lot of threads where the 560 Ti is simply unstable for some reason, but for some others it works fine. I can't really find any good benchmarks comparing the 560 Ti vs. the 580 on Ultra at 1080p. Also, SLI and CF is still somewhat shaky in BF3, but it probably will be sorted out with drivers soon. As of right now I would advise a GTX 580 with a good cooling system over the 560 Ti if you want to avoid any problems and ensure smooth gameplay. My GTX 580 running at 910mhz gets 55-60 fps average on ultra which I feel is the sweet spot. Examples of GTX 580's I would get are MSI Lightning, ASUS Direct CU II, and Galaxy Accelero.